Measurement of acoustic impedance of perforations in contact with absorbing material in the presence of mean flow

The acoustic impedance of perforation in contact with absorbing material in the presence of mean flow is experimentally determined using a modified impedance tube setup. Various perforation samples contacting fibrous material with two different filling densities (100 and 200 kg/m3) are tested for two different flow speeds (Ma= 0.05 and 0.1). The transfer matrix method with predetermined complex values of characteristic impedance and wavenumber of fibrous material are utilized to measure the perforation impedance. The measured resistance and end correction coefficient of the perforation impedance relative to those without flow are presented, thereby illustrating the effect of mean flow on the perforation impedance. The results show that the normalized resistance decreases at low flow speeds and increases at high speeds. The normalized end correction coefficient generally decreases due to the mean flow particularly for small holes at high speeds.